The present invention relates to a magnetic recording medium used in computers and other data-recording and data-processing instruments. More particularly, the invention relates to an improvement in a magnetic recording medium suitable for high-density information recording by virtue of the use of a single crystal wafer of semiconductor silicon as the non-magnetic substrate, on which the magnetic recording layer is formed by the dry-process film-forming method such as sputtering from a ferromagnetic material without using any binder materials and any particulate materials.
Along with the rapid progress in the electronics industry or information-predominating society utilizing computers and other electronic data processing machines, a recent trend in the computer technology is toward a larger and larger capacity of data-recording media. In particular, magnetic recording media or magnetic disks, which are still playing a leading role as an external memory device of computers, are required to have a capacity and a density of recording increasing year by year. This trend of requirements for a higher capacity and higher density of magnetic recording is further accelerated along with the prevalence of very compact personal computers such as so-called notebook-type and palm-top personal computers.
Magnetic recording media in general have a basic structure comprising a fiat and smooth non-magnetic substrate plate usually in the form of a circular disk and a magnetic layer formed on at least one surface of the substrate plate. The substrate plate of a magnetic recording medium must satisfy various requirements in order that the recording medium can exhibit excellent performance. Several of such requirements for the substrate plates, which of course must be non-magnetic, include, for example, high mechanical strengths to ensure easy handling, smoothness of the surface to ensure a high recording density in the magnetic recording layer formed thereon with a small flying height of the magnetic head, good adhesion to the magnetic layer or undercoating layer thereof, stability of the configuration or form against warping or distortion, high abrasion resistance, light weight in order that the magnetic recording disk can be driven with a small power consumption and so on.
Typical materials conventionally used for substrate plates of magnetic recording media are aluminum or alloys thereof and glass. While glass-made substrate plates have a problem in the mechanical strengths, aluminum-made substrate plates have been used heretofore satisfactorily in respect of the above mentioned requirements. Along with the recent progress of the computer technology, however, the substrates of magnetic recording media are required to be further improved in respect of the smaller surface roughness, higher abrasion resistance and lower weight. Turning now to the material for the magnetic recording layer, it is traditional that the magnetic recording layer is formed by coating the surface of a substrate with a coating composition containing ferromagnetic fine particles and an organic resin as a binder of the magnetic particles. For example, U.S. Pat. No 4,689,260 teaches to form elevations of an abrasion-resistant material anchored to the substrate surface on which a magnetic recording layer is formed by coating with a coating composition comprising a binder and magnetic particles
To give a further explanation, while it is desirable that the magnetic recording layer formed on the substrate has a coercive force as high as possible, the magnetic layer of cobalt-based alloys conventionally used for the purpose is imparted with an increased coercive force when the film-forming sputtering process is performed at an increased temperature within a range up to a certain upper limit of the temperature so that the substrate is usually heated at an elevated temperature before formation of the magnetic layer. Since aluminum has a relatively low hardness, it is usual that the surface of an aluminum substrate is provided with an underlayer of nickel-phosphorus (NIP) having a higher hardness, such a NiP-plated aluminum substrate is subject to warping when the substrate is heated due to the great difference in the thermal expansion coefficient between aluminum and NiP and, in addition, NiP becomes magnetic when it is heated at 280.degree. C. or higher. Glass substrates also have a problem of warping when they are heated at an elevated temperature and then cooled because it is usual that a glass substrate plate is used after a tempering treatment forming stressed surface layers.
In this regard, U.S. Pat. No. 4,675,240, which is equivalent to Japanese Patent Kokai 59-96539, proposes to use a silicon wafer as a substrate of a magnetic recording medium, on which an undercoating layer of chromium is first formed by sputtering and then a magnetic recording layer of an iron-cobalt-chromium alloy having a thickness of about 30 nm is formed thereon also by sputtering. Reportedly, the highest coercive force of the magnetic layer thus formed is 1000 oersted. A silicon wafer as a substrate is superior to aluminum substrates in several respects, for example, in the density of 2.3 g/cm.sup.3, which is substantially smaller than that of an aluminum substrate, i.e. 2.7 g/cm.sup.3, higher Vickers hardness than aluminum to ensure higher surface smoothness by polishing and to give a possibility of omitting a high-hardness underlayer of NiP and the much higher melting point of 1420.degree. C. without transition points up to the melting point than 660.degree. C. of aluminum so as to minimize distortion or warping of the substrate plate even when the substrate is heated up to 600.degree. C. or higher in the course of the sputtering treatment for the formation of a magnetic layer thereon, for example, from a ferrite.